The present invention relates to a process for the production of hollow bodies by continuous casting by using a central magnetic field, and the apparatus for carrying out the process.
The process according to the invention may be applied to all metals capable of being continuously cast by means of conventional methods of casting solid bodies, among which mention may be made of aluminum, copper and steels.
Although the process according to the invention may be quite generally applied to the production of hollow bodies having cross-sections of widely varying shapes, the process according to the invention will be applied with a particularly high degree of benefit to the production of hollow bodies of circular cross-section and in particular by operating using rotary continuous casting, in which case the hollow bodies produced may be used for example as blanks with inner and outer skins or surface layers of high quality, for the production of weld-free tubes.
The production of hollow bodies of circular cross-section, that is to say, hollow bodies having a hollow interior which is generally concentric to the outside section, has been the subject of many varied technical disclosures.
Generally, such known processes use a metal cylindrical or cylindrical-conical mandrel, comprising for example copper, which is internally cooled with water and which is disposed coaxially in the interior of the ingot mold or external casting mold. Arrangements are also made to cool the inner wall of the hollow body produced, generally by means of water, after the formation of a solidified surface layer. As the casting operation proceeds, the initially liquid metal solidifies in contact with the mandrel, the leading edge of solidification then progressing radially with respect to the mandrel.
As the solidification process begins from the free surface of the metal bath, it results in all the dross or scum material formed by slags, inclusions or other non-metal particles which are present on the surface of the bath being trapped in the solidified surface layer which forms the inner skin of the hollow body produced, and generally results in an inner skin which suffers from defects, typical incrustations, slags and folds or ripples, which will have to be removed by means of difficult and expensive surface treatments, before the hollow body produced is subsequently put to use.
The inner skin of such products therefore suffers from the same types of defect as are found on the outer skin of solid bodies in conventional casting processes. Such defects are further aggravated by the small amount of space available, which prevents any mechanical apparatus for at least partially removing them, from being introduced.
Certain processes have been developed, in an effort to overcome such difficulties, such as that described in Swiss Pat. No. 618,363 dated Jan. 6, 1977, which uses the electromagnetic effect of an outer single-turn inductor and an inner single-turn inductor to effect continuous casting of hollow bodies, without using an external mold or mandrel.
The inductors used in that process are supplied with a single-phase alternating current and therefore generate a stationary sinusoidal magnetic field which is generally referred to as a pulsating field.
The pulsating field basically promotes the generation of pressure forces within the liquid metal, which move the liquid metal away from the fixed walls in which the inductors are contained, without giving rise to substantial circulatory movements within the mass of liquid metal.
Thus, using that process, a ring of liquid metal is maintained in a condition of equilibrium by a magnetic field, the free surface of the metal being of a convex shape, as shown in FIG. 1 of the above-mentioned patent. Bearing in mind the small radius of action of the magnetic field, that necessarily means that the column of liquid metal is of small height. Such a method can probably be used for aluminium which has a relatively shallow solidification well and a relatively flat solidification leading edge.
By contrast, in the case of steel, being a metal of high density (at least with respect to aluminium) and being a much worse conductor of heat than aluminium, the solidification well, being the distance measured in the bar in the course of solidification from the free surface of the metal bath to the terminal solidification region, is very deep and much greater than that of aluminium. That would result in the necessity for extremely slow casting speeds in order to produce a solidified skin of sufficient strength to contain the metal which is still in a liquid condition, taking account of the pressure forces generated by the pulsating magnetic field, so that that process, assuming that it can be carried into effect, when used with steel, is totally unusable from the economic point of view.
Another way of improving the quality of the inner skin in cast hollow bodies, as described in French patent No. 2 180 494, comprises using a rotary continuous casting process in which a central mandrel is employed, a slag being continuously introduced between the annular surface of the metal in the course of solidification thereof, and the outside surface of the mandrel.
That process suffers from the disadvantage of interfering with heat exchanges and delaying the progression of the solidification leading edge, from the mandrel. In addition, it is necessary to carry out a treatment on the inner surface of the resulting product, before it is used, in order to remove inter alia the layer of slag which is deposited on the inner skin.
Moreover, the general difficulty of the problem to be solved will be noted, bearing in mind the hostile environment: the small amount of space available, both in respect of height and in respect of diameter, at the location of the mold, the danger of an explosion due to the use of water in the event of contact with the liquid metal, particularly when dealing with steel.
Research has therefore been carried out in an effort to find a process for the production of hollow bodies by continuous casting, which does not suffer from the above-indicated disadvantages and which in particular makes it possible to produce hollow bodies, the inner skin of which is of satisfactory quality.
In particular, research has been carried out into the possibility of producing an inner skin of a quality such as to permit hollow bodies to be used without particular surface preparation or with such surface preparation being reduced to a minimum.
The attempt has also been made to find an apparatus for carrying out such a process, which is simple and economical and which can be used for casting many metals or alloys.